1. Field of the Invention
The present invention relates generally to liquid crystals, and, more particularly, to a substrate that pre-aligns a liquid crystal layer.
2. Description of Related Art
One type of liquid crystal electro-optical device is constructed by placing a thin layer of liquid crystal between two transparent plates that have each been coated with a layer of an electrical conductor on its interior face. When no electric field is applied (field-OFF) between the conductive layers, the director of the liquid crystal is in one state. (The "director" of the liquid crystal is the macroscopic direction of the long molecular axis of the liquid crystal molecules.) When an electric field is applied (field-ON), the director reorients to another state. Because the liquid crystal is birefringent, the two states will have different refractive indices, and in many cases the thin layer of liquid crystal will be birefringent for light directed against the transparent plates. The state change responsive to an applied electric field is the basis for liquid crystal devices that control light, such as displays and projectors.
In its usual form, a liquid crystal light valve is a device that modulates a polarized projection light beam directed against one side of the light valve according to a writing light beam directed against the other side. The polarized projection light beam enters the light valve through one of the transparent electrodes, usually termed the counter-electrode, passes through the liquid crystal layer, and is reflected from a mirror on the other electrode. The projection light beam passes back through the liquid crystal layer and the counter-electrode, and through an external analyzer. The polarizing and analyzing functions can often be accomplished by a single polarizing beam splitter through which the projection light beam passes before and after passing through the liquid crystal. The operation of this and other types of liquid crystal light valves is discussed in greater detail in numerous technical publications; see, for example, "Progress in Liquid Crystal Light Valves", by W. P. Bleha, in Laser Focus/Electro-Optics, October 1983, pages 111-120.
In this type of liquid crystal light valve, an electric bias field is applied through the liquid crystal layer by a voltage on the conductive electrodes. The liquid crystal is oriented responsive to this electric bias field. The director of the liquid crystal is initially, in the electric field OFF state, aligned perpendicular to the plates. Application of the electric field causes the director to rotate toward the plane of the plate, changing the light transmittance of the device. The writing light pattern modulates the electric field, changing the phase retardation of the light passing through the liquid crystal, which in turn modulates the projection light beam passing through the analyzer.
For certain applications, it is desirable to orient the directors of these crystals such that they assume parallel alignment with respect to the surface of a substrate. Such an alignment is necessary for the television projection display and color symbology light valves. In general, parallel alignment is desirable if an electric field is applied across the liquid crystal normal to the surfaces, and if the liquid crystal has a positive dielectric anisotropy, so that when the molecules lie parallel to the substrate surface they can be tilted towards a perpendicular orientation.
U.S. Pat. Nos. 4,030,997, issued Jun. 21, 1977, to L. J. Miller et al and 4,022,934, issued May 10, 1977 to L. J. Miller describe methods for aligning liquid crystal molecules in the homeotropic perpendicular or tilted orientation, employing a deposited silicon dioxide which is treated by immersion in a liquid mixture of an aliphatic alcohol and an amine.
There are also methods for aligning the liquid crystal molecules parallel to the surface ("homogeneous" alignment). The simplest method is to rub the surface. Another method is to coat the surface with SiO.sub.2 and etch the surface with an ion beam, as described in U.S. Pat. No. 4,153,529 by M. J. Little et al. Yet another method is oblique deposition of SiO.sub.2 as reported by J. L. Janning, Applied Physics Letters, Vol. 21, pp. 173-174 (1972).
Many of the processes using a deposited SiO.sub.2 layer and treating the SiO.sub.2 layer with an alcohol in the vapor phase require several hours of processing time. The bulk of the processing time is primarily consumed in separate heating cycles for the alcohol source and the substrate on which the alcohol vapors are to be deposited. The reason for this process specification is the need to avoid formation of alcohol condensation droplets on the coated substrate. In the prior art, the separate heating cycles were obtained by placing the substrate in a vacuum chamber provided with a first heating means for raising the temperature of a long chain alcohol and a second heating means for raising the temperature of a liquid crystal cell to a level slightly higher than than of the alcohol. This prior art process is very time consuming, since thermal equilibrium between the alcohol vapors and the heated substrate must be reached before the reaction can take place.
There remains a need for a process which reduces the process time for treating SiO.sub.2 -coatings on substrates. The present invention fulfills this need, and further provides related advantages.